This invention relates to a method and apparatus for reducing or eliminating the effects of noise jamming on coherent radar systems and frequency/phase modulated communication receivers. The invention both enhances the detection of a weak signal in the presence of strong noise jamming, and allows recovery of any frequency and/or phase modulation on the signal. Signal detection is enhanced by a heterodyning or nonlinear processing technique; however, other circuitry is also employed to preserve the frequently and phase information carried by the signal.
Modern coherent radar and communications receivers perform an accurate measurement of signal phase or frequency, and they require that any transmitted phase/frequency modulation be preserved and recoverable in the receiver. For example, in a pulse doppler radar, the target return pulse must be coherent with the transmitted pulse so that the doppler frequency shift of the return signal can be measured. The doppler measurement gives an indication of the target velocity and allows a target to be separated from ground clutter on the basis of differential doppler shifts.
Another example of a coherent type system is a pulse compression radar which transmits a frequency modulated (e.g., FM chirp) or phase-coded RF pulse. The received pulse is decoded and compressed in the receiver by a filter matched to the pulse characteristics. The result of the matched filter/pulse compression processing is improved radar sensitivity and range resolution. Unless the transmitted frequency or phase modulation is preserved on the received signal, the benefits of pulse compression are not realized.
Many modern communication systems use phase shift keying (PSK) and frequency shift keying (FSK) modulation formats. Thus, phase or frequency discrimination is inherent in demodulating the received signal and recovering the data. PSK AND FSK techniques are applied in both direct data transmission systems and spread spectrum systems. In a spread spectrum communication system, the signal spectrum is spread over a wide bandwidth by superimposing a wide bandwidth code on the digital data stream. The spread spectrum receiver collapses the wide signal bandwidth to the data bandwidth by stripping off the code and, as a result of this processing, an improvement in signal-to-noise ratio is achieved. Again, the modulation on the received signal must be undisturbed for the spectrum despreading processing to be effective. Therefore, it is essential that any anti-jam signal processing technique applied to the above systems must preserve the signal phase/frequency information.
Noise jamming from hostile or friendly sources poses a significant threat to coherent radars and communication receivers. A commonly used jamming technique is FM-noise in which an RF carrier is frequency modulated by a baseband noise signal. Spot FM-noise signals with RF bandwidths from 1 MHz to 50 MHz and barrage noise signals with bandwidths from 50 MHz to several hundred MHz are typically used. In some cases noise amplitude modulation may also be intentionally or unintentionally induced on the FM-noise signal.
The effect of noise jamming on coherent systems is to obscure the received signal to the extent that it cannot be detected or demodulated in the receiver. Against a pulse doppler radar system, this jamming can effectively deny target detection, ranging, and doppler frequency measurement. Pulse compression type radars have some inherent protection against noise jamming, but with sufficient power a noise jammer can still effectively mask the radar return, thus prohibiting target detection and ranging. In FSK or PSK communication systems, noise jamming effectively increases the receiver noise level so that the detection sensitivity suffers and the bit error rate or distortion level increases.
In the past, heterodyning or nonlinear processing techniques have been used for suppressing jammer interference in receivers. There are some variations in the techniques, but the basis of all of them is that the jammer is heterodyned or mixed with the desired signal to generate a jammer-signal difference of "beat" frequency signal. This beat frequency component, which has the envelope characteristics of the desired signal, is further processed or demodulated and used as a substitute for the normal receiver output. The heterodyning is performed by a mixer or nonlinear device in which the jamming functions as the mixer local oscillator (LO) signal. With proper filtering of the mixer output signal, an improvement in signal-to-jammer power ratio is obtained.
The fundamental limitation of these techniques is that the frequency/phase information or "coherence" of the signal is destroyed in the heterodyning process. Since the signal is mixed with the jammer, the difference frequency signal has the frequency or spectral characteristics similar to the jammer. Only the signal envelope and timing information is recoverable with these techniques.
Various heterodyne anti-jam techniques have been devised. These include the Counter-Countermeasure Guidance System described in U.S. Pat. No. 3,943,515 and the Radar Device Using Heterodyning Without a Local Oscillator described in copending application Ser. No. 690,423. These particular devices are noncoherent and are not readily applicable to a phase or frequency modulated system.
Another patented approach, described in U.S. Pat. No. 3,949,309, uses a jammer heterodyning technique that preserves the spectral characteristics of a received communication signal. However, the technique is only effective against unmodulated or slowly varying CW jamming and relies on the ability of the receiver to phase-lock an oscillator to the jamming signal. Phase-locking to the jammer is not practical in most cases, especially when the jammer is noise modulated or broadband. Also, narrowband CW jamming is seldom used against radars because of the difficulty involved in setting a narrowband jammer on the radar frequency.
In the present invention, the basic heterodyning anti-jam technique is retained while the frequency/phase modulation on the desired signal is preserved. Herein lies the advantage of this invention over prior art. This capability makes the invention applicable as an anti-jam device for CW and pulse doppler radars, pulse compression radars, frequency/phase modulated communication systems, and radar warning receivers that use frequency discrimination.